why complexity increases over time ?

Software complexity tends to increase over time, there are many reasons for this, some of them are as follows

Features are added to meet user needs, but less or no attention to outdated features which can be removed

Shortcuts taken during development to meet deadlines can cause higher technical-debt

Projects needs to evolve into directions which were not anticipated in original design

Original design drifts as quick fixes and workaround accumulate

Knowledge is lost due to team turnover, understanding of original decisions vanishes

Supporting multiple platforms than initially anticipated

Keeping dependencies updated while maintaining compatibility results in added complexity